Actually you should not try vertex fitting in the FastSim. Everything is already covered by the effective "Vertexing detectors" in the FastSim macro.

RhoCandList Dslist; //This is a list for Ds
RhoCandList Dslist_fitted; //here define a new list!

for (j=0;j<Dslist.GetLength();++j)
{
PndKinVtxFitter vtxfitter(Dslist[j]);
vtxfitter.Fit();
double chi2_vtx=vtxfitter.GetChi2();
if (chi2_vtx>0.){
RhoCandidate *jfit = Dslist[j]->GetFit();
TVector3 jVtx=jfit->Pos();
Dslist_fitted.Add(jfit); //here you should put your fitted candidate into the new list
}
qa.qaComp("Ds_vtx_", Dslist_fitted, ntp2);